Numerical simulations of acoustic isolators in monopole acoustic logging while drilling based on phononic crystal structures

In acoustic logging while drilling (ALWD), it is crucial to have an ALWD isolator to reduce collar waves and to meet the requirement of wideband acoustic measurements. In this work, we proposed a new acoustic isolator based on phononic crystal structures for reducing collar waves, and its acoustic insulation performance is numerically studied through the finite difference simulation. For the typical ALWD tool, the optimized acoustic isolator has eight-section graded grooves and each contains 11 small grooves varying from 3 to 5 mm, with an interval of 0.2 m between adjacent sections. Performances of the isolators we designed are verified by numerical results. The attenuation effects of the improved isolator and the traditional one on collar arrivals are compared. The comparison of synthetic waveforms reveals that the newly proposed design with grooves characterized by gradually varying width displays better acoustic isolation performance than the traditional isolator with uniform grooves when operated with various source central frequencies from 13 to 15 kHz. By applying the proposed isolator, the formation longitudinal wave velocities can be separated effectively from the wave group containing the collar waves in the fast formation model. These studies may be useful for the ALWD tool design and data processing.